This invention relates to the filtering of light containing image-representative information and, more particularly, to an adjustable opto-acoustical low pass filter and technique.
When a solid state sensor, for example in a video camera or a still image digital camera, senses images containing spatial frequencies beyond the Nyquist limit, the electronic video signals from the solid state sensor may produce a displayed picture containing artifacts such as aliasing, moire patterns, and phantom color patterns. An optical low pass filter can be used to suppress high frequency components of the image received by the sensor. However, existing approaches to low pass optical filtering have various disadvantages.
One type of conventional optical low-pass filter is based on birefringent crystal plates. A disadvantage of this type of filter is the requirement for a precise orientation axis of the stacked birefringent crystal plates, making it unsuitable for mass production. Also the number of crystal plates in this design increases the overall length of the optical system. The stack of crystal plates requires a large optical path to separate a passing ray on two or more exiting rays. Another disadvantage of this type of filter is the high cost of the crystal raw material. Still another disadvantage of the birefringent filter is that it depends on the configuration and pitch of the pixels. The birefringent filter is inflexible in that it can be used only with a certain type of solid state sensor.
Another commonly used filter is a phase noise type of optical low-pass filter, which has a periodic structure of phase diffraction grating that can be two dimensional. This filter is thinner than the birefringent filter. The diffraction grating structure depends on the pixel structure. The position of this type of filter relates to the pitch of the diffraction grating and the pitch of the pixels in the solid state sensor. The optical characteristics of the diffraction grating low pass filter are low in quality as compared with the birefringent filter but are significantly less expensive.
It is among the objects of the present invention to provide an improved optical low pass filter and technique that overcomes problems and limitations of prior art approaches.
The present invention is directed to an optical low pass filter and technique that provides excellent performance in conjunction with simplicity and low cost. The filter of the invention has the advantage of flexibility; that is, it can be used with any solid state sensor. Also, the filter is adjustable.
In accordance with an embodiment of the invention, there is disclosed an apparatus for receiving light containing image-representative information, and low-pass filtering said light, the apparatus comprising: a pellicle positionable in the path of said light; at least one transducer coupled with the pellicle; and means for applying an AC electrical signal to the transducer to establish waves in the pellicle; whereby said waves are operative to low-pass filter the image-representative information in light reflected from the pellicle.
In a preferred embodiment of the invention, the waves are ultrasonic waves and the pellicle is a plastic sheet. The pellicle can alternatively be a thin glass sheet.
In a described embodiment, the light is also transmitted through the pellicle, and the pellicle is thin enough to avoid low-pass filtering of the image-representative information in the portion of the light transmitted through the pellicle.
Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.